The present invention relates to a method and a machine for determining at least one space coordinate of a measurement point on a measurement object. More particularly, the invention relates to a method and a machine involving a first and a second movable support for carrying and moving a probe head, wherein the first and second movable supports may have slightly different positions along the axis of movement of the probe head.
Generally, the space (or spatial) coordinates of a measurement point on an object can be determined by approaching the measurement point with a movable probe head of a coordinate measuring machine. In the case of tactile coordinate measuring machines, the measurement point is touched with a touch element (in general a touch pin or stylus). The position of the probe head and, optionally, a deflection of the touch element relative to the probe head allow to determine the desired space coordinates of the measurement point. Similar considerations apply to coordinate measuring instruments with optical or other non-contact probe heads. As will be readily appreciated, the measurement accuracy of the coordinate measuring machine strongly depends on how accurately the respective position of the probe head can be determined when approaching the measurement point.
Measurement errors may result from inaccuracies and/or modifications of the position measuring instruments for the probe head. For example, glass measuring scales are often used as measuring instruments, which are read incrementally when moving the probe head in order to determine the respective travel position of the probe head in space. The glass measuring scales may expand or contract as a function of the temperature.
Another cause of error is due to manufacturing tolerances in the guides of the coordinate measuring machine, i.e. some sort of “waviness” or other deviations from an exactly straight guide path of the probe head in the intended movement direction.
A third cause of errors may be due to elastic deformations of the coordinate measuring machine, as a function of the gravitational moment which the probe head exerts in its respective travel position on the displacement mechanism.
Finally, measurement errors may result from position deviations of the probe head, which arise because of and/or during the travel movements. These include for example oscillations which may be caused by starting and braking the probe head. For coordinate measuring machines in portal or gantry design (i.e. coordinate measuring machines having a movable bridge on two separate supports movable parallel to one another), measurement errors may for example result from movements along the movement axis of the bridge (typically the Y axis), and specifically when there is a lack of synchronicity in the travel movement of the two supports. For example, if the left-hand support is moved further in the Y axis by a distance ΔY than the right-hand support, then this leads to a rotation of the probe head about an orthogonal axis (typically the Z axis). This in turn leads to measurement errors both in the Y direction and in a second orthogonal direction (typically the X axis). In this context, however, it should be emphasized that the invention is not restricted to measurement errors in these special axes. Rather, the invention may be used for all coordinate measuring machines in which a displacement position of the probe head along at least one axis is determined by means of two mutually separated measuring instruments for this one axis.
It has been known for a long time to correct temperature-induced measurement errors by correcting the measurement values derived from the measuring instruments as a function of the respective ambient temperature. For example, the temperature-dependent length changes of glass measuring scales can be corrected by means of thermal expansion coefficients. It is also known to correct geometrical guide errors and elastic deformations by means of correction values which are recorded by a calibration run of the probe head for various probe head positions, and are provided in a memory. Such correction methods are often referred to as CAA (computer aided accuracy) in the specialist terminology, and the Assignee of the present invention offers a program named LASERCAL for recording calibration data and for appropriately correcting measurement deviations.
DE 102 14 490 A1, for instance, discloses a method for correcting elastic guide errors in a horizontal-arm coordinate measuring machine.
The known methods, however, are not capable of correcting measurement errors, which are the result of irreproducible variations in the measurement operation, with the desired accuracy. These errors include inter alia rotary measurement errors of coordinate measuring machines in portal or gantry design as a result of oscillations and/or due to lack of synchronicity in movements along the Y axis.
DE 22 48 194 B2 proposes to record the respective displacement positions of the portal feet of a coordinate measuring machine in portal design by two separate measuring instruments. A difference value is formed from the measurement values obtained. This difference value is subsequently used for correcting the measurement value delivered by one of the measuring instruments. No distinction is made between whether the difference value was caused by reproducible or irreproducible variations.
Another method of this kind is known from EP 0 309 094 A1. In this case as well, no distinction is made between reproducible (for example due to geometry) and irreproducible (dynamically caused) deviations and resulting measurement errors.
DE 29 50 926 A1 discloses a coordinate measuring machine in which three measuring instruments, spatially separated from one another, are used for each movement axis in order to correct measurement errors. Difference formation is employed for the correction in this case as well, but without distinguishing between measurement errors of different causes.
Finally, EP 0 537 641 B1 proposes to correct the measurement errors of a coordinate measuring machine in portal design during a travel movement along the Y axis, by recording the deviations in the position measurement values of two separate measuring instruments during a time period before the sampling, and by determining a correction value for the position of the sampling element at the sampling time from the time profile of the stored measurement values. It is thereby possible to correct dynamic measurement errors due to oscillations. This method, however, is restricted to the use of switching probe heads. In this form, it cannot be used with measuring probe heads.
Even though the last mentioned methods already provide higher measurement accuracies with coordinate measuring machines in portal or gantry design than without computational correction, the measurement accuracy is not yet optimal.